Dual-center thermochromic Bi2MoO6:Yb3+, Er3+, Tm3+ phosphors for ultrasensitive luminescence thermometry

Abstract

Optical thermometers are of great interest due to their non-contact, high-sensitivity and fast measurement characteristics. In this work, a series of dual-center Bi1.96−xMoO6: 0.02Er3+, 0.02Tm3+, xYb3+ (x = 0.10–0.35) upconverting materials were prepared by a sol-gel synthesis method. Upon 975 nm excitation, the prepared materials exhibit bright color-tunable (from yellow to orange) upconversion (UC) emissions, as the Yb3+ content increases. The thermometric properties of the synthesized materials associated with different thermally-coupled and non-thermally coupled levels of Tm3+ and Er3+ were systematically investigated. Based on the temperature-dependent emissions originating from the non-thermally coupled levels of Tm3+ (3F2,3) and Er3+ (4F9/2), i.e., their band intensity ratios 700/670 nm, the developed optical thermometers were found to exhibit an exceptional relative thermal sensitivity (Sr), up to 5.90% K−1 at 293 K. Importantly, in the whole T-range of 293–623 K, the Sr values are larger than 2% K−1. Furthermore, it is revealed that the position of the Tm3+ emission band, centered around 800 nm is highly dependent on temperature, and, so, it can be utilized as a second thermometric parameter, which is important for a multi-parameter temperature sensing in the T-range of 293–623 K. These results suggest that Er3+/Tm3+/Yb3+-doped Bi2MoO6 materials are promising candidates for ultra-sensitive, dual mode optical thermometers and safety sign applications.